Sen. Bernie Sanders (I-Vt.) is probably best known in Washington for two things. One -- his status as the Senate's only self-identified socialist -- is unique. The other, a bit less so: as even his friends would admit, Bernie Sanders can be kind of a grump -- a charter member of the "get off my lawn" caucus.

The presidential primary challenger to Hillary Clinton is regularly drawing crowds in the thousands for his message decrying income inequality, and his calls for a more even distribution of the nation's wealth. Ask him about the minimum wage. Ask him about Wall Street malfeasance. But whatever you do, don't ask him about, say, the way he likes his coffee. Bernie Sanders isn't big on small talk.

That confession is one of the ways the septuagenarian has reminded us of our own grumpy grandfathers. Here are seven others:

1. He gets impatient with millennial email things

In Wednesday's Skimm -- a wildly popular morning newsletter catered toward millennial woman, for those of you who live under a rock -- Sanders agreed to an interview. He made it clear when that interview was over.

(but 404 pages he'll have a little fun with. Just don't stick around too long, or he might get cranky.)

3. He's always right

Sanders is kind of known for cutting off interviewers. Here he is in 2014 objecting to National Journal reporter Simon Van Zuylen-Wood's assessment that "old white guys" building the 2015 progressive base runs counter to where Democrats want to take their party:

"Who told you that?" he scoffs. "I'm talking from a little bit of experience. I did get 71 percent of the vote in my state. And despite popular conception—with all due respect to my friends in California, Northern California, where you have wealthy liberals who support me and I appreciate that—Vermont is a working-class state. So I'm glad you raised that, because your analysis is incorrect. And I'm right and everybody else is wrong. Clear about that?"

In 1979, Sanders produced a 28-minute audio documentary on Eugene Debs, a union leader and five-time candidate for president under the socialist ticket. The New Yorker's Jill Lapore dug up "Eugene v. Debs: Trade Unionist, Socialist, Revolutionary, 1855-1926." Here's how it begins:

"It is very probable, especially if you are a young person, that you have never heard of Eugene Victor Debs. If you are the average American, who watches television forty hours a week, you have probably heard of such important people as Kojak and Wonder Woman, have heard about dozens of different kinds of underarm spray deodorants, every hack politician in your state, and the latest game between the Boston Red Sox and the New York Yankees. Strangely enough, however, nobody has told you about Gene Debs, one of the most important Americans of the 20th century."

All this, people who have known him for decades say, is the real Bernie Sanders. And that "grumpy old man" demeanor actually dates back to his earliest days in politics. Here's Boston Globe's Christopher Rowland on Sanders:

"When I was a reporter for the Brattleboro Reformer in 1988, I covered Sanders on a campaign swing through southern Vermont as he was waging a losing campaign for US House (he won in the next election cycle, in 1990). He was completely humorless, relentlessly focused on the evils of the American corporation. Not self-important, but self-serious to an extreme....

....Yes, those of us who covered him back in the 1980s can attest — this is the real Bernie. He has been on message, it seems like, forever.."